The present invention relates to a holographic display where a colour reconstruction of a three-dimensional object is generated by way of space division colour multiplexing, and where this reconstruction is visible from visibility regions.
Document DE 10 2004 044 111 filed by the applicant describes a holographic display device for generating a monochromatic holographic reconstruction of a three-dimensional object (3D object). The present invention is based on that display. It comprises as main components light source means, focussing optical means, a controllable spatial light modulator (SLM) and an image separation means, which is realised for example in the form of a lenticular.
To maintain a certain clarity of the diagram, only two monochromatic pixels of a pixel matrix of the controllable light modulator SLM and only one separating element of the image separation means L are shown in the top view in FIG. 1a. The separating element is here a cylindrical lens of a lenticular. Two separate, column-wise interleaved one-dimensional holograms of the 3D object, which reconstruct a left and a right view of that object, respectively, are written to the SLM. Two pixel columns of the SLM are covered by one cylindrical lens, where the cylindrical lenses are arranged in parallel to the pixel columns. The arrangement of the pixel columns, which are denoted with l and r, is shown schematically in a front view in FIG. 1b. The pixel pitch which is relevant for the encoding is denoted with SP.
The cylindrical lenses image the pixel columns l and r into an observer plane. In synchronism with that, the focusing optical means also image light which is coherent in the vertical direction after the modulation in the SLM into the observer plane, where the light is superposed there to form visibility regions VWL and VWR for a left and right observer eye, respectively. The visibility regions VWL and VWR are diamond-shaped three-dimensional regions each of which forming an intersecting plane with the observer plane. The observer eyes must lie in that intersecting plane between two diffraction orders of the light used in order to be able to perceive the holographic representation of the 3D object. The light which falls on the pixels, as indicated by an arrow in the drawing, and which is spatially coherent in the vertical direction and spatially incoherent in the horizontal direction is emitted e.g. by line light sources.
The light source means and the focussing optical means, i.e. a lenticular, which serves as wave-optical means for realising the Fourier transformation (FT) at the same time, both of which being disposed upstream the SLM in the optical path, are not shown in FIGS. 1a to 1c so to maintain a certain clarity of the drawing.
According to the prior art, in order to generate a colour reconstruction which comprises the primary colours RGB, an individual hologram is computed for each colour and all three holograms are displayed on the SLM either simultaneously or sequentially. To realise a simultaneous display of the holograms by way of space division multiplexing, these holograms are spatially interleaved. Three primary colours and two observer windows thus mean that a six-fold multiplexing is required. The SLM is simultaneously illuminated with light emitted by RGB light sources, and standard colour filters which are accordingly assigned to the pixels of the SLM filter out the light for the respective pixels.
FIG. 1c shows schematically the front view of the standard arrangement of the pixel columns l and r of FIG. 1b for a colour 3D presentation. In this arrangement, three rectangular sub-pixels of the primary colours R, G and B together form a square pixel. The column denoted with l is imaged to the left visibility region VWL, and the column denoted with r is imaged to the right visibility region VWR. Three sub-pixels with the integrated colour filters R, G and B form a square pixel with a certain pitch, denoted with 3SP, which is relevant for the 1D encoding. 1D encoding here means that the hologram has a vertical movement parallax. Because only light of identical wavelength is able to interfere, the pitch 3SP which is relevant for the encoding is three times as large as in a monochromatic 3D presentation. The vertical extent of the visibility region is thus only ⅓ as large as in a monochromatic display.
FIG. 2a shows a different possibility of generating a colour holographic 3D presentation using an SLM with integrated RGB colour filters. They are assigned to two sub-pixel columns per colour and three cylindrical lenses of the image separation means L in the order RRGGBB (see FIG. 2b). Further, the two sub-pixel columns per colour comprise a left and a right hologram for that colour. Referring to FIG. 2a, a complete superposition of the sub-pixel images is realised, thereby forming visibility regions VWL and VWR which are larger than in the previously discussed example. The pitch SP which is relevant for the encoding is here again large enough to get visibility regions of same vertical extent as in a monochromatic display according to FIG. 1b. However, SLM with such a sequence of colour sub-pixels are not commercially available and can thus not be used for holographic reconstruction methods. The colour R, G or B is not indicated in each of the pixels in FIG. 2b in order to keep up clarity of the drawing.
Generally, the colour filters can be applied external to the cover glass of the SLM. However, this has the disadvantage that there will be a gap between the pixel arrangement and the colour filters which corresponds to the thickness of the glass plate of about 1 mm when applying the colour filters external to the cover glass of the SLM. Therefore, and because of the substantially smaller pixel pitch (<100 μm) of the colour sub-pixels, there are diffraction effects which cause under certain circumstances a disturbing crosstalk between neighbouring sub-pixels. Further crosstalk occurs if the optical path runs through the SLM and colour filter at an angle other than a right angle. This will be the case if the visibility regions are tracked to the observer by way of displacing the light sources. For an observer who is situated substantially off the central axis of the display and who perceives the light at an oblique angle, the light of a hologram which has been computed for a certain colour does not pass through the respective colour filter. That observer would therefore perceive a defective reconstruction. In the above-discussed example with a thickness of the cover glass of 1 mm and a pitch of 100 μm, this will be the case for angles greater than 6°, while a gradual deterioration already takes place at smaller angles. The viewing angle at which the display can be used is thus substantially limited, and the display cannot be used by multiple users.
These disadvantages are particularly grave when manufacturing prototypes or small series of holographic displays, because commercially available SLM panels or external colour filters must be used then. For this reason, it makes sense not to realise the colour presentation with the help of the SLM, but with the help of the optical means in the display device.
It is known from the prior art to realise the colour presentation in a display device with the help of colour filters in conjunction with optical imaging means, e.g. micro-lenses.
Document WO 99/50914 describes how coloured micro-lenses focus light which is emitted by a large light source on a small sensitive region, which can be a sensor or pixel. The function of a micro-lens is here combined with that of a colour filter in the form of a monochromatic micro-lens. The micro-lenses are given their shape with a curved surface which realises a certain inclination angle for the desired emission of monochromatic light during the manufacturing process. The LC molecules of a subsequently disposed LC layer, which are hit by monochromatic light in a small region, serve as switches and filter out the light according to the colour of the filter in that region. A processor combines three monochromatic pencils of rays to form one colour pixel, thus defining its colour and intensity. Colour display devices can thus be made with monochromatic micro-lenses, where one colour pixel is always represented by a group of three such monochromatic micro-lenses. This arrangement is suitable for colour representations with ordinary commercially available colour display panels. However, because the light is focussed on the pixels, that type of colour display is not suited for the above-mentioned holographic reconstruction principle. Additional imaging means had to be disposed in the optical path in order to focus the light on observer eyes. This would increase the structural depth and the weight of a flat display, which, however, shall be avoided.
Document U.S. Pat. No. 5,682,215 relates to a colour display with an array of micro-lenses where each micro-lens is tinted with one of the colours RGB. Thanks to the colour filtering, the coloured micro-lenses realise two functions on the light that passes through them. It is focussed specifically on one pixel in order to improve the brightness of the display and the aperture ratio. The micro-lenses are here tinted for example with colour pigments. Again, that colour display can only be used in a holographic display device and with a holographic reconstruction method as described above in conjunction with additional optical components for generating a visibility region.